Photomasks for the electronics industry must be perfect and have high resolution. For TV picture tube shadowmasks, which are large in scale, about 2 feet by 3 feet (about 61 by 91 cm) and which have hundreds of thousands of metal dots deposited on the glass surface, the photomask must have no missing or malformed dots, since these result in corresponding imperfections on the cathode ray viewing screen which are visible and annoying to the viewer.
One well known method of making photomasks is to electrolessly deposit nickel alloy dots on the glass. These photomasks have satisfactory resolution and good abrasion resistance. However, it has proven to be almost impossible to make a photomask which does not have some missing or malformed dots. Generally, each mask has about 6 - 10 defects.
In the past, making or repairing the metal dots has been accomplished by either a subtractive or a semi-additive process.
U.S. Pat. No. 3,833,375 illustrates a subtractive process whereby a layer of nickel is electrolessly deposited on the surface of the photomask and the unwanted portions are removed using conventional photolithographic and etching techniques. This process has several disadvantages; it is wasteful of metal; and when the unwanted nickel is etched away, poor edge definition can occur due to undercutting and incomplete etching.
Feldstein in U.S. Pat. No. 3,753,816 discloses a method of repairing metal dots by depositing a relatively thin, etchable nickle film on the glass substrate, which metal is catalytic to the deposition of a second metal film, applying a photoresist over the first metal film, exposing and developing it to form the desired pattern, depositing a second, relatively thick and relatively unetchable metal film over the now exposed metal areas, then removing the photoresist and etching way the etchable metal film not covered by the second metal film. This process is only semi-additive in that some metal film must be etched away, although the metal dots themselves are put down only where they are wanted.
It would be desirable to apply the metal dots where wanted without having to apply unwanted metal and to have to etch it away. However, attempts to apply a photoresist directly onto the photomask surface, expose and develop the photoresist to leave openings where metal is to be deposited, and electrolessly plate metal in the openings, have proven entirely unsatisfactory because the metal will not plate directly on the exposed surface.